In one embodiment, the present invention relates to a blank made of titanium-doped silica glass for a mirror substrate for use in EUV lithography, with a surface portion to be provided with a reflective film and having an optically used area CA over which the coefficient of thermal expansion CTE has a two-dimensional dCTE distribution profile averaged over the thickness of the blank with a maximum inhomogeneity dCTEmax of less than 5 ppb/K defined as a difference between a CTE maximum value and a CTE minimum value.
Furthermore, in one embodiment, the present invention relates to a method for producing such a blank from titanium-doped glass (quartz glass) having a high silicic-acid content for a mirror substrate for use in EUV lithography.
In EUV lithography, highly integrated structures with a line width of less than 50 nm are produced by means of microlithographic projection devices. Use is here made of working radiation from the spectral range between 10 nm and 121 nm, which is also called EUV range (extreme ultraviolet light, also known as “soft X-ray radiation”). The typical operating wavelength in EUV lithography is 13 nm at the moment.
The projection devices are equipped with mirror elements which consist of synthetic glass having a high silicic-acid content and being doped with titanium dioxide (hereinafter also called “TiO2—SiO2 glass”, “Ti-doped silica glass”), and which are provided with a reflective layer system. These materials are distinguished by an extremely low coefficient of thermal expansion (shortly called “CTE” hereinafter; coefficient of thermal expansion). The CTE is a glass property which depends on the thermal history of the glass and on a few other parameters, but mainly on the titanium dioxide concentration. Typical titanium dioxide concentrations are between 6% by wt. and 9% by wt.
The blank of Ti-doped silica glass is mechanically treated to obtain the mirror substrate and is provided with a reflective film to form a mirror element. The mirror element has an optically used area CA (CA stands for “clear aperture”) which is specified for each mirror element and impinged by a predetermined radiation profile when used in an EUV projection system. Also in a mirror element blank which is not provided with a reflective film, CA is already defined as a projection onto the blank surface, namely independently of the treatment condition of the blank (before or after mechanical treatment).
The optical specification of the area CA of the optical blank is a design factor which is comparable to other technical design factors in the art, e.g. the load waterline (LWL) which characterizes the length of a boat where it meets the water when loaded to its designed capacity. The design factor is utilized throughout the design and manufacturing of the respective product in order to characterize how well it meets certain performance requirements. It is used to specify the acceptable limits of key parameters that govern system performance, as well as to specify the amount of resources (i.e. time and cost) that must be spent on manufacturing in order to reach the specification.
The maximum (theoretical) reflectivity of such an EUV mirror element is about 70%, so that at least 30% of the radiation energy is absorbed in the mirror coating and in the near-surface region of the mirror substrate and converted into heat. Viewed over the total volume, this therefore yields an inhomogeneous temperature distribution with a temperature difference that under typical operating conditions may amount up to 50° C. and lead to a deformation of the mirror substrate.
To keep this deformation small, it would therefore be desirable that the mirror substrate within a significant volume has a CTE that is around zero throughout the total operating temperature range. In practice, however, it is difficult to satisfy this condition because for a given glass composition the temperature range with a CTE around zero is small.
The temperature at which the CTE is exactly equal to zero is also called temperature of zero crossing or TZC (temperature of zero crossing). This glass property also depends substantially on the titanium concentration. The concentration is typically adjusted such that the CTE is about zero in the temperature range between 20° C. and 45°.
To reduce imaging errors caused by inhomogeneous temperature distribution in the mirror substrate blank, it is provided in International Application Publication No. WO 2011/078414 A2 that in a blank for a mirror substrate, the concentration of titanium oxide over the thickness of the blank is adapted stepwise or continuously to the temperature distribution occurring during operation, in such a manner that the condition for the zero crossing temperature TZC is satisfied at every point, i.e., the coefficient of thermal expansion for the locally evolving temperature is substantially equal to zero. This is said to be accomplished in that during production of the glass by flame hydrolysis, the concentration of start substances containing titanium or silicon, respectively, is varied such that a predetermined concentration profile is set in the blank.
It is evident that the reproducible adjustment of a variable, location-dependent titanium dioxide concentration over the volume of the mirror substrate blank is complicated and can be optimized also only for one of many mirrors of the projection device and only for a single specific irradiation constellation.
Another approach is taken in DE 10 2004 024 808 A1, which discloses a quartz glass blank for a mirror substrate of the aforementioned type and a method for the production thereof. The blank of titanium-doped, synthetically produced quartz glass known therefrom is given a cylindrical shape; it has, e.g., a diameter of 300 mm and a thickness of 40 mm. This yields by way of a mechanical treatment a mirror substrate plate which is provided with a reflective film completely or in part.
It is suggested that local homogeneity variances in the coefficient of thermal expansion (hereinafter also called “CTE inhomogeneity” or shortly “dCTE” and defined as the amount of the local deviation from an absolute minimum value CTEmin of the CTE distribution profile: (dCTE=CTE−CTEmin)) should be accepted under certain conditions in favor of a less expensive production method. According to these conditions, the dCTE does not exceed a predetermined limit value and in a measurement over the optically used area CA, it shows an evolution that can be described sufficiently accurately through a small number of low-frequency spherical Zernike polynomials. To be more specific, the known quartz glass blank has the following properties:
a) it contains micro-inhomogeneities caused by local variance of the TiO2 distribution of less than 0.05% TiO2,
b) it shows a radial variance of the thermal expansion coefficient over the usable area CA of not more than 0.4 ppb/(K·cm),
c) its absolute maximum inhomogeneity in the thermal expansion coefficient dCTEmax on the optically used area CA (averaged over the thickness of the blank) is less than 5 ppb/K,
d) where the dCTE evolution over CA can substantially be described by the following Zernike terms:
                    dCTE        =                              C            0            α                    +                                    C              3              α                        ⁡                          (                                                2                  ⁢                                                            (                                              r                        R                                            )                                        2                                                  -                1                            )                                +                                    C              8              α                        ⁡                          (                                                6                  ⁢                                                            (                                              r                        R                                            )                                        4                                                  -                                  6                  ⁢                                                            (                                              r                        R                                            )                                        2                                                  +                1                            )                                                          (        1        )            
where: C0α<=5 ppb/K; r=radial distance from the cylinder axis; R=radius of CA; and C3; C8=adaptation parameters of the terms.
The phrase “can substantially be described by the following Zernike terms” means that after subtraction of the terms, the remaining residual inhomogeneity of the thermal expansion is not more than 0.5 ppb/K (in the original document “dCTE” is designated as “Δα”).
U.S. Patent Application Publication No. 2010/0003609 A1 describes the preparation of a Ti-doped glass for photomask substrates for EUV lithography. The substrates show a radial refractive index distribution profile in which preferably the maximum is in the center, even if the blank has polygonal cross section. The polygonal cross-section is produced by way of “outflowing a molten glass mass” of a round rod in a polygonal shape.
DE 42 04 406 A1 describes the preparation of a striae-free body by multi-step forming process comprising a step in which a molten glass mass flows out in a rectangular shape. In this case, an intermediate product having a rectangular cross section is obtained which is finally twisted so as to obtain a rod with round cross section.
DE 10 2013 101 328 B3 describes a blank made of TiO2—SiO2 glass for a mirror substrate for use in EUV lithography. At a mean value of the fictive temperature Tf in the range between 920° C. and 970° C., the glass shows a dependence of its temperature zero crossing TZC on the fictive temperature Tf that, expressed as differential quotient dTzc/dTf, is less than 0.3.
DE 10 2010 009 589 A1 discloses a method for producing a blank made of Ti-doped silica glass for a mirror substrate for use in EUV lithography, wherein a soot body of titanium-doped SiO2 is produced by means of flame hydrolysis of silicon- and titanium-containing start substances, and whereby the soot body is dried in vacuum by heating to a temperature of at least 1150° C., so that a mean hydroxyl group content of less than 150 wt. ppm is set, the dried soot body is sintered forming a preform of Ti-doped silica glass, and the Ti-doped silica glass is loaded by way of the conditioning process with hydrogen, so that a mean hydrogen content of at least 1×1016 molecules/cm3 is obtained.
CTE inhomogeneities that satisfy these conditions show a substantially rotation-symmetrical distribution profile over CA. A mirror substrate blank adapted in this way tolerates comparatively high absolute CTE inhomogeneity values also in case of inhomogeneous heating if the irradiation profiles occurring during use and thus the optically used area CA show a circular symmetry. The known mirror substrate blank is however not suited for use with irradiation profiles having a geometry differing from the circular form.
It is an objective of the present invention to provide a blank of titanium-doped silica glass for a mirror substrate for use in EUV lithography, which blank also without an individually adapted, location-dependent adjustment of the titanium dioxide concentration is designed and suited for use with a non-circular irradiation profile. Furthermore, it is an objective of the present invention to provide a method which permits the reproducible and inexpensive production of such a mirror substrate blank.